Microorganisms that are currently being used to ferment sugars to biofuels such as ethanol usually cannot utilize complex polysaccharides such as cellulose and hemicellulose. As a result, a significant bottleneck occurs in the conversion of lignocellulosic materials to biofuels.
Cellulose, a major component of plants and one of the most abundant organic compounds on earth, is a polysaccharide composed on long chains of β(1-4) linked D-glucose molecules. Due to its sugar-based composition, cellulose is a rich potential source material for the production of biofuels. For example, sugars from cellulose may be fermented into biofuels such as ethanol. In order for the sugars within cellulose to be used for the production of biofuels or other commodity chemicals, the cellulose must be broken down into smaller molecules.
Cellulose may be enzymatically hydrolyzed by the action of cellulases. Cellulases include endoglucanases, exoglucanases, and beta-glucosidases. The actions of cellulases cleave the 1-4 β-D-glycosidic linkages in cellulose, and result in the ultimate release of β-D-glucose molecules. During the breakdown of cellulose into individual sugar molecules, glucose polymers of various lengths may be formed as intermediate breakdown products. Glucose polymers of approximately 2-6 molecules in length derived from the hydrolysis of cellulose are referred to as “cellodextrins” or “cellooligosaccharides.”
Hemicellulose constitutes the second largest component of polysaccharides in many plants, such as the perennial grasses switchgrass and Miscanthus. Hemicellulose is a complex polysaccharide that has a xylose-linked backbone, with side chains of arabinose, glucuronyl, and acetyl groups. A structural model of a hemicellulose illustrates the xylose backbone residues joined together in beta-1,4-linkages (FIG. 1). Several functional groups decorate the backbone, including esters of acetyl (Ac) groups, arabinose, glucuronic acids, and esters of feroryl groups. The feroryl groups link the entire structure to lignin. Enzyme cocktails that hydrolyze hemicellulose into its major component sugars such as xylose (a 5-carbon sugar) and arabinose (a 5-carbon sugar) will significantly increase the fermentable sugars for biofuel production from lignocellulose-based feedstock. Enzymatic removal of hemicellulose by hemicellulases will also increase accessibility of cellulases to the cellulose component of plant cell walls or lignocellulosic feedstocks. Thus, the degradation of hemicellulose is a critical step in the utilization of lignocellulose feedstock for biofuel production.
Thermostable enzymes are particularly desirable for the efficient degradation of cellulose and hemicellulose, because thermostable enzymes are more compatible than non-thermostable enzymes with other processes involved in converting lignocellulose-based materials into biofuels. For example, treatments of lignocellulose-based materials to decrease the crystallinity of cellulose may require high temperatures that inactivate non-thermostable enzymes.
In addition, thermostable enzymes are desirable for the degradation of cellulose and/or hemicellulose because they may have a higher specific activity as compared to their mesophilic counterparts, and because they can operate at high temperatures that reduce or eliminate the risk of microbial contamination.
Accordingly, there is a need for thermostable enzymes and enzyme cocktails capable of degrading cellulose and/or hemicellulose.